The present invention relates to a method of manufacturing an optical semiconductor element and, more particularly, to a method of manufacturing an optical semiconductor element having an epitaxially grown ridge.
As an optical amplifier, a rare-earth (Er, Nd, or the like) added optical fiber amplifier, and a semiconductor laser amplifier (to be abbreviated to as an LD amplifier hereinafter) are expected to be useful in optical communications. Of these amplifiers, an LD amplifier especially has compactness so that it should be appropriate to compact integrated components, and can cope with any wavelengths for optical communications. Thus, various techniques associated with LD amplifiers have been developed, and the following technical reports have been announced.
1) K. Magari et. al. IEEE photonics Technology Letters Vol. 2 (1990) pp. 556 PA1 2) M.S. Lin et. al. IEEE Journal of quantum Electronics Vol. 26 (1990) pp. 1,772-1,778 PA1 3) I. Cha et. al. Electronic Letters Vol. 25 (1989) No. 18 pp. 1,241-1,242 PA1 4) S. Cole et. al. Electronics Letters Vol. 25 (1989) pp. 314-315 PA1 5) N. A. Olsson Electronics Letters Vol. 25 (1989) pp. 1,048-1,049
When an LD amplifier is put into a practical application, the improvement of the gain, a decrease in dependency of the gain on polarization of incident light (polarization dependency; a TE-TM gain difference), and a decrease in ripple of the gain depending on the wavelength of incident light should be attained. In the references 1 and 2, the polarization dependency can be almost eliminated, and a sufficient internal gain (up to 25 dB) is obtained. In the reference 3, the ripple of the gain is almost eliminated.
The polarization dependency of the gain in the LD amplifier is eliminated by adopting a strained quantum well structure (reference 1), a buried double-hetero structure having an active layer with a small width (up to 4,000 .ANG. or less) (reference 2), or a structure having a thick active layer (reference 4). However, for these conventional structures, there are the following problems. In the LD amplifier using the strained quantum well structure of the reference 1, it is difficult to grasp the proper composition of the layers, and to control the composition on the manufacturing. Therefore, the demand has arisen for a method allowing easier manufacturing.
The LD amplifier having the narrow active layer of the reference 2 has a difficulty in its manufacturing process, and it is difficult to form this LD amplifier with good reproducibility. When the narrow active layer is formed, a compound semiconductor including the active layer is incised into a narrow (4,000-.ANG. wide) mesa-stripe shape. It is very difficult to realize this incision process by a conventional wet etching process with good reproducibility.
In the LD amplifier having the thick active layer of the reference 4, since the injected carrier density in the active layer is decreased upon driving of the LD amplifier, a decrease in gain inevitably occurs. For the above-mentioned causes, it is not easy to manufacture an LD amplifier, which has a small polarization dependency, and can provide a sufficient gain (up to 20 dB or higher), by the conventional technique.
As a problem on the characteristics of the LD amplifier, the ripple of the gain must be considered. The ripple is caused by reflection of signal light propagating through a waveguide in the amplifier on the facet of an LD amplifier element.